1. Field of the Invention
The present invention relates to an image forming apparatus capable of optically detecting a toner image that is formed on an image bearing member for performing adjustment, and adjusting image forming conditions based on the detection result.
2. Description of the Related Art
There is a method performed in an image forming apparatus for processing various recording materials. Such an image forming apparatus includes an intermediate transfer member to which a toner image to be used for performing adjustment (hereinafter, referred to as an “adjustment toner image”) is transferred from a photosensitive member. The image forming apparatus then forms the adjustment toner image on the photosensitive member and detects the adjustment toner image on the photosensitive member to adjust the image forming conditions.
A detection unit irradiates the adjustment toner image on the photosensitive member with light to detect the adjustment image. A potential of an irradiated portion on the adjustment toner image which has been irradiated with light thus locally shifts to the positive polarity on the photosensitive member. As a result, if a voltage of positive polarity is further applied when the irradiated portion passes through a transfer unit, the potential of the irradiated portion may be reversed from the negative polarity to the positive polarity, and may have an influence on the image.
Japanese Patent Application Laid-Open No. 2007-286445 discusses a configuration in which an optical sensor that detects the adjustment toner image is positioned opposite to the image bearing member. In such a configuration, when the region irradiated with light on the adjustment toner image in the image bearing member passes through a transfer unit, a voltage of negative polarity and greater than a discharge start voltage is applied to the transfer unit. A trace of the optical sensor irradiating the adjustment toner image with light is thus reduced.
However, a length of the adjustment toner image may be set long to increase accuracy of image density adjustment. In such a case, if the method discussed in Japanese Patent Application Laid-Open No. 2007-286445 is employed, the following problem occurs. More specifically, if a voltage which is higher than the discharge start voltage is applied to the transfer unit, the potential of the photosensitive drum changes. As a result, if the length of the adjustment toner image is longer than a circumferential length of the photosensitive drum, a portion of the adjustment toner image is formed on the photosensitive drum under a different potential, due to discharge in the transfer unit. The density of such a portion thus becomes lower. In other words, the portion of the adjustment toner image is formed under a different condition, so that density unevenness is formed on the adjustment toner image. As a result, it may affect density adjustment using the adjustment toner image.
To solve such a problem, if the length of the adjustment toner image is longer than the circumferential length of the photosensitive drum, it is desirable to set the voltage to be applied to the transfer unit as follows. That is, the voltage to be applied to the transfer unit is of negative polarity and less than the discharge start voltage while the region irradiated with light passes through the transfer unit. However, if the length of the adjustment toner image becomes longer, the optical sensor repeatedly irradiates the photosensitive drum with light to detect the adjustment toner image. The potential of the region in the photosensitive drum which the optical sensor has repeatedly irradiated with light becomes close to a ground potential. If the photosensitive drum is then exposed by a discharging device in such a state, it becomes difficult for photo-carriers generated on the photosensitive drum as a result of exposing by the discharging device to be used in a discharging process. The photo-carriers thus do not immediately disappear, and remain on the photosensitive drum. If the image forming process is performed while the photo-carriers remain on the photosensitive drum, the traces may be generated in subsequent images.